Problem: Nadia is 3 times as old as Kevin and is also 20 years older than Kevin. How old is Kevin?
We can use the given information to write down two equations that describe the ages of Nadia and Kevin. Let Nadia's current age be $n$ and Kevin's current age be $k$ $n = 3k$ $n = k + 20$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $k$ , and both of our equations have $n$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3k$ $-$ $ (k + 20)$ which combines the information about $k$ from both of our original equations. Solving for $k$ , we get: $2 k = 20$ $k = 10$.